Combine the brine ingredients. Soak salmon in brine for 15 minutes.
While the salmon is brining, combine all ingredients for the blackened seasoning.
When the salmon is almost finished brining, heat the avocado oil in a 12 inch skillet over medium high heat.
Drain the salmon and pat dry with paper towels. It is important to get the salmon dry or the skin won't crisp.
Sprinkle 1/4 tsp kosher salt evenly over the salmon skin, then press it into the skin with your fingers. Sprinkle the blackened seasoning over the salmon flesh, then press it into the salmon with your fingers. Do not put the blackened seasoning on the skin, as it could burn during cooking.
When the avocado oil is shimmering, place the salmon in the skillet with the skin side down. Immediately do two things: weigh the salmon down with a cake pan or another skillet that has something heavy in it AND lower the heat to medium low.
Cook the salmon skin side down until the skin is crispy, the salmon releases from the pan easily, and the flesh is cooked nearly to the top of the salmon, approximately 7-9 minutes
Flip the salmon and cook until it reaches an internal temperature of 120°F-130°F, about 1 minute.
Serve immediately with the skin side up.
Notes
Using thicker salmon filets will enable you to get the crispiest skin before the salmon is cooked all the way through. With thinner filets it is more difficult to achieve extra crispy skin without overcooking the salmon.